This invention relates generally to frequency shifters and particularly to optical frequency shifters. Still more particularly, this invention relates to a guided wave optical frequency shifter, which is suitable for use in an optical rotation sensing system. The frequency shifter uses electro-optic rotating half wave plate to shift the frequency of light input to a frequency suitable for the angular rotation rate to be detected.
A frequency shifter may be used to implement an optical rotation sensing system having the bandwidth required for aircraft navigation. Such systems must be capable of resolving rotation rates as low as 0.01 degrees per hour and as high as 1000 degrees per second. The ratio of the upper limit and the lower limit to be measured is approximately 10.sup.9.
Fiber optic ring interferometers have proven to be particularly useful for rotation sensing. A fiber optic ring interferometer typically includes a loop of fiber optic material having counterpropagating light waves therein. After traversing the loop, the counterpropagating waves are combined so that they interfere to form an output signal. The intensity of the output signal is dependent upon the relative phase of the two waves. Rotation of the loop creates a relative phase difference between the two waves in accordance with the well-known Sagnac effect. The amount of phase difference is a function of the angular velocity of the loop so that the optical output signal produced by the interference of the counterpropagating waves varies in intensity as a function of the rotation rate of the loop. Rotation sensing is accomplished by detecting the optical output signal and processing the signal to determine the rotation rate. A frequency shifter may be included in the apparatus that processes the optical output signal to determine the amount of the Sagnac phase shift between the two waves.
The frequency shifter must have a bandwidth comparable to that of a suitable light source used to introduce counterpropagating light waves in the loop. A superluminous diode having coherent light output in a wavelength band of about 100 nm has been found to be a suitable light source for an optical rotation sensing system. Therefore, a suitable frequency shifter must be capable of shifting an input optical beam by at least 50 nm above and below the carrier wavelength. Previously available frequency shifters have bandwidths that are too narrow for aircraft guidance systems.
A typical optical frequency shifter uses an acoustic wave to form a moving grating. The light be to be shifted in frequency impinges upon the acoustic wave. The alternating regions of condensation and rarefaction in the acoustic wave act as a diffraction grating having a grating spacing equal to the wavelength of the acoustic wave. The grating is moving at the acoustic wave velocity and shifts the incident optical beam in frequency by means of the Doppler effect. The acoustooptic frequency shifter has a bandwidth of only about 2.ANG.=0.2 nm. Acoustooptic frequency shifters typically have efficiencies of about 80%, but the efficiency varies with the frequency and the amount of frequency shift. A frequency shifter in an optical rotation sensing system should have a conversion efficiency that is invariant with respect to changes in the amount of frequency shift required.